The present invention relates generally to planar antennas and, more particularly, to a planar antenna arrangement defining at least one pair of dominant paths carrying antenna currents and having a configuration disposed therebetween for producing additional antenna currents. The planar antenna of the present invention is advantageously implemented in a bow-tie configuration.
Planar antennas are used at microwave, millimeter wave, and infrared frequencies to couple energy between free space and a wire circuit. The planar configuration of these antennas enables ease of fabrication using electrically conductive layers formed on non-electrically conductive substrate materials. The antenna itself includes the electrically conductive layer sitting atop the substrate layer which, of course, exhibits a dielectric constant (defined as permittivity relative to the permittivity of free space). The high dielectric constants of familiar substrates such as, for example, silicon, act in an adverse manner by degrading the efficiency of reception, or in other words, “gain”, of a signal arriving at the antenna from a direction opposite the substrate. This side is typically referred to as the “air-side” of the antenna. The gain is mathematically defined as the product of air-side directivity and radiation efficiency. Directivity represents the preferential radiation in a particular direction over all other directions. Radiation efficiency is the ratio of power radiated by the antenna to the power lost due to various mechanisms including metal losses and surface waves.
Generally, antenna currents induced from air-side radiation create high electric fields within such a high permittivity substrate, which, in turn, cause a leakage of energy into the substrate, partly in the form of surface wave modes. Further, surface waves induced by discontinuities in the substrate and traveling along the plane of the antenna, interfere with expected antenna reception. At least the aforementioned phenomena, relating to substrate properties, contribute to limited air-side gain.
The prior art has attempted to cope with limited air-side gain in a number of ways. For example, in one approach for increasing air-side gain, the conductive layer of the antenna is sandwiched between a prism and the substrate to couple energy into the antenna more efficiently, as is taught in a paper entitled “Coupling to an ‘edge metal-oxide-metal’ junction via an evaporated long antenna” by Y. Yasuoka et al.[1]. Unfortunately, however, at 10 μm wavelengths and below, planar antennas are themselves only several microns in length. Accordingly, attachment of such lens components is submitted to necessitate additional fabrication steps, which may well prove to be tedious and expensive.
In another approach that is described in a paper by T.-L. Hwang, et al.[2], a dielectric layer (called a superstrate) is deposited onto the air-side of the antenna with the intention of increasing antenna fields in the air-side direction. However, the reception in this case is enhanced in the plane of the antenna, and not in the air-side direction perpendicular to the antenna. For applications where the radiation is incident from the air-side direction perpendicular to the substrate, this antenna configuration of Hwang et al. will not be suitable.
Alternatively, the bottom of the substrate may be coated with a conductive layer which will reflect substrate energy leakage back towards the conductive metal layer of the antenna, as discussed in a paper by C. Fumeaux, et al. (hereinafter Fumeaux) [3] for radiation at a wavelength of 10.6 μm. At shorter wavelengths below 10 μm, however, this approach is submitted to become unreliable due to variability in the thickness of the substrate. For example, a typical silicon substrate includes a thickness of 385 μm+/−2 μm. For wavelengths on the order of 2 μm, this uncertainty in the thickness will cause unpredictable reflections by the additional conductive layer.
In still another approach, described in a paper entitled AN EFFICIENT ULTRA-WIDE BOW-TIE ANTENNA by A. A. Lestari et al. (hereinafter Lestari)[4], a bow-tie antenna is proposed which minimizes late time ringing resulting, at least in part, from internal reflections in the context of a ground penetrating radar. Such ringing is disadvantageous since it may mask radar targets. The antenna proposed by Lestari is shown in FIG. 3 of the paper, including a slot grating formed on the bow-tie configuration in attempting to reduce reflections from the open end of the bow-tie arm. Accordingly, the curved Lestari slot grating extends completely widthwise across each antenna bow arm between its diverging pair of edges. The described slot configuration is perceived by Lestari as advantageous specifically in the way that it interferes with the native, dominant bow-tie currents by causing these currents to produce radiation before reaching the open end of the bow-tie arm. This result is clearly seen in FIG. 4 of Lestari. As will be further described, the present application considers this interfering configuration as being disadvantageous.
Other approaches to the problem have encompassed the use of planar log periodic antennas. One form of planar log periodic antenna is the toothed log periodic bow-tie antenna, as described, for example, in a reference by Stutzman et al.[5]. This configuration includes slots cut into the diverging outer edges of each bow-tie antenna arm. Again, as will be further described, the present application considers this configuration as disadvantageous since native bow-tie currents are significantly altered. In the log periodic antenna and the toothed bow-tie antenna, the presence of the slots in the dominant current path cause the antenna currents to completely decay before arriving at the edges of the antenna. This avoids any resonant effects and gives rise to a broadband antenna. Insofar as applications where a resonant antenna may be preferable to keep out radiation at unwanted wavelengths, the log periodic is therefore undesirable.
The present invention provides a highly advantageous planar antenna including a grating or slot configuration which resolves the foregoing problems while providing still further advantages.